Agent dispensing systems, manufactured in the form of dispensing devices, for the precision administration of agents with controlled delivery patterns and with extended operational delivery times are known to the dispensing art in the U.S. Pat. Nos. 3,845,770 and 3,916,899, both issued to inventors Theeuwes and Higuchi, and in the U.S. Pat. No. 4,350,271 issued to inventor Eckenhoff. In these patents, the dispensing devices disclosed comprise a semipermeable wall that surrounds a compartment containing an agent formulation that is dispensed from the device under the influence of (1) an osmotically active solute, or (2) an osmotically active swellable polymer. These dispensing devices are extraordinarily effective for delivering an agent that possesses degrees of solubility, from poorly soluble to very soluble, in aqueous and biological fluids.
The above dispensing devices represent outstanding and pioneering advancements in the dispensing art, and they are useful for dispensing innumerable agents to various environments of use. Now, it has been discovered these dispensing devices can be improved further by enhancing the agent delivering kinetics and the usefulness of the devices. That is, it has now been discovered unexpectedly that dispensing systems can be provided that initially deliver bio-affecting agent followed by a substantially constant amount of agent at a controlled rate over time; thereby making agent instantly available to an agent receptor by substantially eliminating the startup agent delivery time frequently required to deliver agents from these dispensing devices. The dispensing systems made available by the invention embody a unique initial agent delivery followed by controlled and constant prolonged delivery, thereby functioning according to a preselected, built-in optimal program of agent presentation.